Investment casting, often referred to as “lost wax casting,” is a casting process that enables fabrication of near-net-shape metal parts from metals and metal alloys. Investment casting is typically used to form intricate, complex shapes with high precision. Refractory crucibles are used in investment casting processes for melting metal alloys. In this respect, alloys are melted in a crucible and then poured from the crucible into a mold to form a cast shape. Conventional refractory crucibles, such as zirconia crucibles, typically have a porous structure, which in part, increases resistance to thermal shock and which further minimizes the potential for cracking.
Certain metal alloys are more reactive than others with refractory materials and can react with the inner surface of the crucible. Nickel-based and cobalt-based superalloys that have high chrome concentrations and titanium alloys are examples of more reactive alloys. These reactive alloys tend to mechanically and/or chemically react with the refractory material, e.g., zirconia, which forms the crucible. With repeated use, the interaction between the alloy and refractory material can result in deterioration, i.e., erosion and corrosion, of the inner surface of the crucible. The molten alloy can penetrate into the porous surface of the crucible and react with the refractory material. Moreover, the penetration of the alloy into the porous surface of the crucible can further cause deterioration of the surface as the metal within the surface repeatedly solidifies and re-melts between melt cycles. The repeated melting and solidifying of the metal within the refractory material can cause cracking and spalling of the refractory and subsequent crumbling of the refractory at the surface of the crucible. This not only reduces the life of the crucible, but may also result in inclusions of refractory material in the cast part(s).
The present invention overcomes these and other problems and provides a refractory crucible having a dense inner surface structure that is less susceptible to penetration of alloys, and a method of impregnating the surface of a crucible.